(1) Field of the Invention
The present invention relates to a radial tire having an improved durability, and particularly relates to an improvement in a radial tire provided with a belt having a folded portion of metal cords.
(2) Description of the Prior Art
In the belt of radial tire, unextendable metal cord, particularly steel cord, is preferably used as a reinforcing material, and belts having the following structures are known; a structure wherein a plurality of rubberized cord layers, each containing cords arranged at a small inclination angle of 5.degree.-25.degree. with respect to the equatorial plane of the tire, are merely superposed such that the cords arranged in adjacent layers are crossed with each other; and a so-called fold structure, wherein one of adjacent two layers of a plurality of layers is folded at the end portions in the width direction, and the folded layer and a layer which is not folded are superposed in various arrangements, or both the adjacent layers are folded at the end portion in the width direction, and are superposed with each other.
The fold structure in a belt is higher in the rigidity in the circumferential direction than a structure wherein cord layers are merely superposed with each other, due to the fold effect of the cord layer. Therefore, the tire having the fold structure ought to be excellent in the cornering stability and in the high-speed durability. However, in the tire, the inclination angle of the cord with respect to the equatorial plane is small, and therefore extremely high tension acts locally on the cords located at the bent corner formed by the folding of the cord layer, and the cords are apt to break not only by an action of external force during the running, but also even by an internal pressure applied to the tire during the vulcanization of the tire. Therefore, a satisfactory improvement of durability of tire has not yet been attained.
There have been proposed the following methods as a means for solving the above described drawbacks.
1. A method disclosed in Japanese Patent Laid-Open Application No. 65,602/73, wherein a cord layer is folded in a direction, which decreases the twist of cords, to decrease the excess torsion in the folded portion.
2. A method disclosed in Japanese Patent Laid-Open Application No. 44,902/74, wherein one or more of auxiliary layers are inserted into the inside of the folded portion to form a large radius R of curvature in the folded portion, whereby the bending strain is decreased.
However, in the above described method (1), the bending strain cannot be decreased; and in the method (2), the bending strain can be decreased, but the thickness of the belt layer becomes very large, and therefore the resulting tire is poor in the resistance against heat built-up and in the high-speed durability.